1. Field of the Invention
The present invention relates to a hardenable composition which comprises an acrylate and/or methacrylate monomer (hereinafter referring to as (metha)acrylate monomer), a hardener and a hardening accelerator.
2. Description of the Prior Art
It has been known that the hardenable compositions comprising the (metha)acrylate monomer, a hardener and a hardening accelerator are useful for liquid sealants, adhesive compositions and paints.
These hardenable compositions were disclosed in U.S. Pat. No. 2,895,950 Japanese Patent Publication No. 3595/1963 and Japanese Unexamined Patent Publication Nos. 33234/1975; 64326/1975 and 129632/1975. The hardeners in the compositions are catalysts for polymerizing the (metha)acrylate monomer and various organic peroxides or hydroperoxide have been used as the hardener.
It is optimum to select a catalyst having fast hardening rate i.e. catalyst imparting fast polymerization rate. Among known hardeners, benzoyl peroxide and cumene hydroperoxide are preferably used. However, it is not suitable to use a hardener which decomposes in a storage to form radicals and to polymerize the (metha)acrylate monomer in the contact with the (metha)acrylate monomer even though it imparts fast polymerization rate. For example, benzoyl peroxide easily decomposes to form radicals when it is stored or placed in a hot condition, whereby the storage stability is too low in the case of the mixture of benzoyl peroxide and (metha)acrylate monomer.
When the (metha)acrylate monomer is polymerized with benzoyl peroxide, the impact strength of the hardened product is relatively low and local hardening is caused and it is not suitable for bonding broad area. When benzoyl peroxide is contacted with a skin, skin inflammation is caused.
When cumene hydroperoxide is mixed with the (metha)acrylate monomer, it is relatively stable in storage whereby the hardening is not easily caused and the hardening is initiated by adding a hardening accelerator. However, cumene hydroperoxide causes bad smell, skin diseases and respiratory organ diseases by breathing. Accordingly, it has been required to find an improved hardener from the environmental pollution and hygiene.